1. Field of the Invention
The present invention relates to an improvement of a power transmission unit, and particularly to a power transmission unit connected to a gear box such as a differential gear of a four-wheel drive vehicle.
2. Description of the Prior Art
Generally, front wheels of a four-wheel drive vehicle are driven directly by a driving source while rear wheels thereof are driven via a power transmission unit utilizing the so-called viscous coupling which transmits power by the viscous resistance of fluid.
The viscous coupling transmits very small torque when a rotational difference between an input shaft and an output shaft is small, while it transmits very large torque when the rotational difference is large.
FIG. 1 shows an example of the viscous coupling adapted for a power transmission unit according to the prior art.
In this figure, an input side of the coupling is on the right-hand side, and an output side thereof on the left-hand side. A flange yoke 101 connected to a propeller shaft on the input side is fixed to an input shaft 103 by bolts 105. Engaged with the periphery of the input shaft 103 is an end of an outer cylinder 107 which is fixed thereto solidly by welding, etc.
On the inner surface of the outer cylinder 107, there are provided a plurality of grooves 109 (for instance, grooves for a spline joint) in parallel with an axis of the outer cylinder 107. Engaged with the grooves 109 are teeth 113 of input plates 111 acting as resistance plates The teeth 113 (for instance, teeth for the spline joint) are formed on the periphery of each of the input plates 111 to engage with the grooves 109 provided for the outer cylinder 107 such that the input plates 111 do not rotate with respect to the outer cylinder 107. The inner diameter of each input plate 111 is formed slightly greater than the circumference of a sleeve 117 engaged with an output shaft 115 such that the input plate 111 does not interfere with the periphery of the sleeve 117.
The input plates 111 mentioned above and output plates 119 to be described later are arranged alternately one after another and spaced apart from each other by a predetermined distance "f". At an end of the combination of the input plates 111 and the output plates 119, there is disposed a flange 121 which is engaged with the outer cylinder 107. A stopper 123 is arranged on the inner surface of the outer cylinder 107 to stop the axial movement of the flange 121. The input shaft 103, outer cylinder 107, and flange 121 constitute an input member. The input member is rotatably supported by a ball bearing 125 and a roller bearing 127 with respect to the sleeve 117 engaged with the output shaft 115 solidly. To prevent the leakage of viscous fluid filled in a working chamber 129 accommodating the resistance plates 111 and 119, and to prevent the leakage of lubricant such as grease filled in the bearings, various seal members 131, 133, and 135 are assembled to the input shaft 103 and flange 121.
On the output shaft side, the periphery of the sleeve 117 engaging with the output shaft 115 (partly shown in the figure) is provided with a plurality of grooves 137 (for instance, grooves for a spline joint) which extend in parallel with an axis of the output shaft 115. The grooves 137 are arranged in a range corresponding to a range in which the grooves 109 of the outer cylinder 107 are disposed. Engaged with the grooves 137 are teeth 139 of the output plates 119 acting as the resistance plates. The teeth 139 (for instance, teeth for the spline joint) are formed on an inner diametral portion of each of the output plates 119. The periphery of the output plate 119 is formed slightly smaller than the inner diameter of the outer cylinder 107 of the input member such that the output plate 119 does not interfere with the outer cylinder 107.
The output plates 119 and the input plates 111 are arranged alternately one after another and spaced from one another by a distance "f".
The working chamber 129 is sealed by the seal members 131 and 135 filled with viscous fluid having high viscosity. The viscous fluid fills the respective gaps "f" between the input and output plates.
The viscous coupling with the above arrangement is adapted as a power transmission unit and disposed between the input propeller shaft and the differential unit for rear wheels to connect them with each other. A drive pinion disposed at an end of the output shaft 115 is connected to a ring gear of the differential unit for rear wheels. At an opening 143 of a housing 141 of the differential unit for rear wheels, there is fitted a seal member 145 which is arranged slidably around the output shaft 115 to prevent the leakage of the lubricant filled in the housing 141 of the differential unit for the rear wheels.
According to the above-mentioned power transmission unit, the input plates 111 arranged for the input shaft 103 transmit power to the output plates 119 through the viscous fluid of high viscosity sealed in the working chamber 129. Accordingly, slippage is caused between the plates and the viscous fluid, heating the viscous fluid.
The generated heat is dispersed to the atmosphere and cooled thereby because the power transmission unit is arranged outside the carrier 141. However, the bearings 125 and 127 and the seal members 131 and 133 arranged on the right and left sides to rotatably support the input member with respect to the output shaft 115 may be deteriorated quickly and cause insufficient lubrication because the heat generated as mentioned above may adversely affect lubricant such as grease filled in the bearings and the seal members. It may be possible to refill the grease if the grease is deteriorated, but it will be a bothersome work.
FIG. 2 shows another example of a power transmission unit according to a prior art ("4.times.4 Magazine", Sept., 1985, page 54, Four by Four Magazine Co.). Since the constitution of a viscous coupling shown in FIG. 2 is the same as that of the one shown in FIG. 1, the explanation thereof will be omitted. In this example, a viscous coupling 205 is assembled in a housing 203 of a differential unit 201 for front wheels. Power is transmitted from an input shaft 207 through resistance plates 209 to an output shaft 211, and, through a ring gear 215 engaging with a drive pinion 213 disposed at an end of the output shaft 211, the differential unit 201 for front wheels is rotated to turn left and right wheel shafts.
According to the above arrangement, respective rotating portions are lubricated by lubricant filled in the housing 203 so that the above-mentioned problem may not be caused. However, heat generated between the output and input resistance plates 209 of the viscous coupling 205 is cooled by the lubricant in the housing 203 of the differential unit 201 for the front wheels. Therefore, heat discharge is not sufficient, causing the heat not to be cooled but stored. As a result, the performance of the transmission unit is unstable, and the sufficient performance is not realized.